Semiconductor device and forming method thereof

ABSTRACT

A method of forming a semiconductor device includes forming a fin structure having a stack of alternating first semiconductor layers and second semiconductor layers over a substrate, the first semiconductor layers and the second semiconductor layers having different compositions, forming a dummy gate structure across the fin structure, forming gate spacers on opposite sidewalls of the dummy gate structure, respectively, removing the dummy gate structure to form a gate trench between the gate spacers, removing portions of the first semiconductor layers in the gate trench, such that the second semiconductor layers are suspended in the gate trench to serve as nanosheets, forming a first titanium nitride layer wrapping around the nanosheets, wherein an atomic ratio of titanium to nitrogen of the first titanium nitride layer is less than 1, and forming a metal fill layer over the first titanium nitride layer.

BACKGROUND

Transistors are components of modern integrated circuits. To satisfy thetrend of increasingly faster speed, the drive currents of transistorsneed to be increasingly greater. To achieve this increase inperformance, the gate lengths of transistors are scaled down. Scalingdown the gate lengths leads to undesirable effects known as“short-channel effects,” in which the control of current flow by thegates is compromised. Among the short-channel effects are theDrain-Induced Barrier Lowering (DIBL) and the degradation ofsub-threshold slope, both of which result in the degradation in theperformance of transistors.

For example, multi-gate devices have been introduced in an effort toimprove gate control by increasing gate-channel coupling, reduceOFF-state current, and reduce short-channel effects (SCEs). One suchmulti-gate device is horizontal gate-all-around (HGAA) transistor, whosegate structure extends around its horizontal channel region providingaccess to the channel region on all sides or three sides. The HGAAtransistors are compatible with complementary metal-oxide-semiconductor(CMOS) processes, allowing them to be aggressively scaled down whilemaintaining gate control and mitigating SCEs. However, fabrication ofthe HGAA transistors can be challenging. For example, nanosheetformation of HGAA transistors by the current methods is not satisfactoryin all respects, especially when using a single process, such as asingle epitaxial process.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 to FIG. 16 are exemplary sequential processes for manufacturingthe gate-all-around (GAA) FET device according to some embodiments ofthe present disclosure.

FIG. 17A to FIG. 21B are various cross-sectional views of a GAA deviceat different stages of fabrication, according to some embodiments of thedisclosure.

FIG. 22 is a chart illustrating flat band voltage (Vn) versuscapacitance equipment thickness (CET) with respect to a work functionlayer in FIGS. 19A and 19B and a reference titanium nitride layer.

FIG. 23 is an atomic model of the work function layer in FIGS. 19A and19B.

FIG. 24 shows a schematic diagram of a sequence of a plasma enhancedatomic layer deposition (PEALD) process according to some embodiments ofthe disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. In certain embodiments,the term “about” used in this context means greater or less than thestated value or the stated range of values by a percentage such as 5%,10%, 15%, etc. of the stated values.

The gate all around (GAA) transistor structures may be patterned by anysuitable method. For example, the structures may be patterned using oneor more photolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers may then be used to pattern the GAA structure.

Reference is now made to FIG. 1 to FIG. 16, which are exemplarysequential processes for manufacturing the gate-all-around (GAA) FETdevice according to some embodiments of the present disclosure. It isunderstood that additional operations can be provided before, during,and after processes shown by FIG. 1 to FIG. 16, and some of theoperations described below can be replaced or eliminated, for additionalembodiments of the method. The order of the operations/processes may beinterchangeable.

Referring to FIG. 1, impurity ions (dopants) 102 are implanted into asubstrate 100 to form a well region. The ion implantation is performedto prevent a punch-through effect. In some embodiments, the substrate100 may include in its surface region, one or more buffer layers (notshown). The buffer layers can serve to gradually change the latticeconstant from that of the substrate to that of the source/drain regions.The substrate 100 may include various regions that have been suitablydoped with impurities (e.g., p-type or n-type conductivity). The dopants102 are, for example, phosphorus for a p-type Fin FET.

Referring to FIG. 2, stacked semiconductor layers are formed over thesubstrate 100. The stacked semiconductor layers include firstsemiconductor layers 110 and second semiconductor layers 112. Further, amask layer 120 is formed over the stacked layers.

The first semiconductor layers 110 and the second semiconductor layers112 are made of materials having different lattice constants, and mayinclude one or more layers of Si, Ge, SiGe, GaAs, InSb, GaP, GaSb,InAlAs, InGaAs, GaSbP, GaAsSb or InP. In some embodiments, the firstsemiconductor layers 110 and the second semiconductor layers 112 aremade of Si, a Si compound, SiGe, Ge or a Ge compound. In FIG. 2, fivelayers of the first semiconductor layer 110 and five layers of thesecond semiconductor layer 112 are disposed. However, the number of thelayers are not limited to five, and may be as small as 1 (each layer)and in some embodiments, 2-10 layers of each of the first and secondsemiconductor layers are formed. By adjusting the numbers of the stackedlayers, a driving current of the GAA FET device can be adjusted.

The first semiconductor layers 110 and the second semiconductor layers112 are epitaxially formed over the substrate 100. In some embodiments,the bottommost first semiconductor layer 110 (the closest layer to thesubstrate 100) is thicker than the remaining first semiconductor layers110.

In some embodiments, the mask layer 120 includes a first mask layer 122and a second mask layer 124. The first mask layer 122 is a pad oxidelayer made of a silicon oxide, which can be formed by a thermaloxidation. The second mask layer 124 is made of a silicon nitride (SiN),which is formed by chemical vapor deposition (CVD), including lowpressure CVD (LPCVD) and plasma enhanced CVD (PECVD), physical vapordeposition (PVD), atomic layer deposition (ALD), or other suitableprocess. The mask layer 120 is then patterned into a mask pattern byusing patterning operations including photo-lithography and etching.Next, as shown in FIG. 3, the stacked layers of the first and secondsemiconductor layers 110, 112 are patterned by using the patterned masklayer, thereby the stacked layers are formed into fin structures 130extending in the X direction. In FIG. 3, two fin structures 130 arearranged in the Y direction. But the number of the fin structures is notlimited to, and may be as small as one and three or more. In someembodiments, one or more dummy fin structures are formed on both sidesof the fin structures 130 to improve pattern fidelity in the patterningoperations.

Referring to FIG. 4, after the fin structures 130 is formed, aninsulating material layer 140 including one or more layers of insulatingmaterial is formed over the substrate so that the fin structures 130 arefully embedded in the insulating material layer 140. The insulatingmaterial for the insulating material layer 140 may include siliconoxide, silicon nitride, silicon oxynitride (SiON), SiOCN, SiCN,fluorine-doped silicate glass (FSG), or a low-K dielectric material,formed by LPCVD (low pressure chemical vapor deposition), plasma-CVD orflowable CVD. An anneal operation may be performed after the formationof the insulating material layer 140. Then, a planarization operation,such as a chemical mechanical polishing (CMP) method and/or an etch-backmethod, is performed such that the upper surface of the uppermost secondsemiconductor layer 112 is exposed from the insulating material layer140. In some embodiments, a first liner layer 142 is formed over thestructure of FIG. 3 before forming the insulating material layer 140.The first liner layer 142 is made of SiN or a silicon nitride-basedmaterial (e.g., SiON, SiCN or SiOCN).

Then, as shown in FIG. 5, the insulating material layer 140 (as shown inFIG. 4) is recessed to form an isolation insulating layer 144 so thatthe upper portions of the fin structures 130 are exposed. With thisoperation, the fin structures 130 are electrically insulated from eachother by the isolation insulating layer 144, which is also referred toas a STI structure. In some embodiments, the insulating material layer140 is recessed until the bottommost first semiconductor layer 110 isexposed. The first semiconductor layers 110 are sacrificial layers whichare subsequently partially removed, and the second semiconductor layers112 will serve as channel regions of a GAA FET.

After the isolation insulating layer 144 is formed, a sacrificial gatedielectric layer 150 is formed, as shown in FIG. 6. The sacrificial gatedielectric layer 150 includes one or more layers of insulating material,such as a silicon oxide-based material. In one embodiment, silicon oxideformed by CVD is used.

Afterwards, a sacrificial gate layer and a mask layer (e.g., having apad SiN layer and a silicon oxide mask layer) are formed over thesacrificial gate dielectric layer 150, followed by patterning the masklayer, the sacrificial gate electrode layer and the sacrificial gatedielectric layer 150 into the sacrificial gate structure 160, as shownin FIG. 7. The sacrificial gate structure 160 includes the sacrificialgate dielectric layer 150, the sacrificial gate electrode layer 164(e.g., poly silicon), the pad SiN layer 166 and the silicon oxide masklayer 168. The stacked layers of the first and second semiconductorlayers 110, 112 are partially exposed on opposite sides of thesacrificial gate structure 160, thereby defining source/drain (S/D)regions. In this disclosure, a source and a drain are interchangeablyused and the structures thereof are substantially the same.

Referring to FIG. 8, after the sacrificial gate structure 160 is formed,a blanket layer 170 of an insulating material for sidewall spacers isconformally formed by using CVD or other suitable methods. The blanketlayer 170 is deposited in a conformal manner so that it is formed tohave substantially equal thicknesses on vertical surfaces, such as thesidewalls, horizontal surfaces, and the top of the sacrificial gatestructure. In some embodiments, the blanket layer 170 is deposited to athickness in a range from about 2 nm to about 10 nm. In one embodiment,the insulating material of the blanket layer 170 is a siliconnitride-based material, such as SiN, SiON, SiOCN or SiCN andcombinations thereof.

The blanket layer 170 is then etched using an anisotropic process toform gate sidewall spacers 172 on opposite sidewalls of the sacrificialgate structure 160 and fin sidewall spacers 174 on opposite sidewalls ofthe fin structures 130, followed by etching exposed portions of the finstructures 130 that extend laterally beyond the gate sidewall spacers172. The resulting structure is illustrated in FIGS. 9A and 9B, whereinFIG. 9B is the cross sectional view corresponding to line X1-X1 of FIG.9A. In FIG. 9B, the cross section of the bottom parts of one sacrificialgate structure 160 is illustrated. In some embodiments, the anisotropicprocess can be control such that no fin sidewall spacers 174 remain onthe STI region 144.

The anisotropic etching performed on the blanket layer 170 can be, forexample, reactive ion etching (RIE). During the anisotropic etchingprocess, most of the insulating material is removed from horizontalsurfaces, leaving the dielectric spacer layer on the vertical surfacessuch as the sidewalls of the sacrificial gate structures 160 and thesidewalls of the exposed fin structures 130. The mask layer 168 may beexposed from the sidewall spacers.

Subsequently, as shown in FIGS. 10A and 10B, the first semiconductorlayers 110 are horizontally recessed (etched) so that the secondsemiconductor layers 112 laterally extend past opposite end surfaces ofthe first semiconductor layers 110. In some embodiments, as shown inFIG. 10B, end surfaces of the first semiconductor layers 110 may besubstantially vertically aligned with the side surfaces of thesacrificial gate electrode layer 164. Here, “substantially verticallyalignment” means the horizontal offset is less than about 1 nm.

During the recess etching of the first semiconductor layers 110 asillustrated in FIGS. 10A and 10B, the second semiconductor layers 112may be also horizontally etched. The recessed amount of the firstsemiconductor layers 110 is greater than the recessed amount of thesecond semiconductor layers 112. In this way, the resulting secondsemiconductor layers 112 can laterally extend past opposite end surfacesof the first semiconductor layers 110.

After the first semiconductor layers 110 are horizontally recessed, aninner spacer 180 is formed on the recessed surfaces of the first andsecond semiconductor layers 110, 112, as shown in FIGS. 11A and 11B.Formation of the inner spacer 180 includes depositing an inner spacermaterial layer (e.g., silicon nitride), followed by etching back theinner spacer material layer by an anisotropic etching process, to removethe inner spacer material layer from the substrate 100.

In some embodiments, the inner spacer 180 includes insulating materialsuch as silicon nitride or the like.

After the inner spacer 180 is formed, source/drain (S/D) epitaxiallayers 190 are epitaxially grown from the exposed recessed fins 165between the fin sidewall spacers 174, as shown in FIG. 12. The S/Depitaxial layer 190 includes one or more layers of Si, SiGe or Ge for ap-channel FET. The S/D layers 190 are formed by an epitaxial growthmethod using CVD, ALD or molecular beam epitaxy (MBE). In someembodiments, the epitaxial layers 190 grown from neighboring recessedfins 165 of the substrate 100 merge above the STI 144 and form a void insome embodiments. In some other embodiments, the epitaxial layers 190grown from neighboring recessed fins 165 do not merged.

Subsequently, a second liner layer 192 is formed and then an interlayerdielectric (ILD) layer 194 is formed, as shown in FIG. 13. The secondliner layer 192 is made of a silicon nitride-based material, such asSiN, and functions as a contact etch stop layer in the subsequentetching operations. The materials for the ILD layer 194 includecompounds comprising Si, O, C and/or H, such as silicon oxide, SiCOH andSiOC. Organic materials, such as polymers, may be used for the ILD layer194.

As shown in FIG. 13, after the ILD layer 194 is formed, a planarizationoperation, such as CMP, is performed, so that the top portion of thesacrificial gate structure 160 is exposed.

Next, as shown in FIG. 14, the sacrificial gate electrode layer 164 (seeFIG. 9B) and sacrificial gate dielectric layer 150 (see FIG. 9B) areremoved, thereby exposing the fin stack of the first and secondsemiconductor layers 110, 112 and a gate trench is formed between thegate spacers.

The ILD layer 194 protects the S/D structures 190 during the removal ofthe sacrificial gate structures. The sacrificial gate structures can beremoved using plasma dry etching and/or wet etching. When thesacrificial gate electrode layer 164 is polysilicon and the ILD layer194 is silicon oxide, a wet etchant such as a TMAH solution can be usedto selectively remove the sacrificial gate electrode layer 164. Thesacrificial gate dielectric layer 150 is thereafter removed using plasmadry etching and/or wet etching.

After the sacrificial gate structures are removed, the firstsemiconductor layers 110 (as shown in FIG. 14) in the fin structures areremoved, as shown in FIGS. 15A and 15B. That is, the first semiconductorlayers 110 (see FIG. 14) are etched. As a result, portions of the secondsemiconductor layers 112 are suspended. In the following discussion, theportions of the second semiconductor layers 112 suspended are alsoreferred to as and serve as the channel layers (or nanosheets). Thechannel layers 112 are slightly etched or not etched. In the presentembodiments, the channel layers 112 are slightly etched to form arectangular-like shape (e.g., a nanosheet). FIG. 15B is the crosssectional view along the fin structure. Gaps 115 are left betweenneighboring channel layers 112. The first semiconductor layers 110 canbe removed or etched using an etchant that can selectively etch thefirst semiconductor layers 110 at a faster etching rate than etching thesecond semiconductor layers 112. The channel layers 112 extend in theX-direction above the substrate 100 and are arranged in the Z directionperpendicular to the X-direction.

In some embodiments, the first semiconductor layers 110 (also calledsacrificial layers to be removed) are SiGe and the second semiconductorlayers 112 (also called channel layers to be left in final GAAtransistors) are silicon allowing for the selective removal of the firstlayers 110. In some embodiments, the selective wet etching includes anAPM etch (e.g., ammonia hydroxide-hydrogen peroxide-water mixture). Insome embodiments, the selective removal includes SiGe oxidation followedby a SiGeO_(x) removal. For example, the oxidation may be provided by O₃clean and then SiGeO_(x) removed by an etchant such as NH₄OH thatselectively etches SiGeO_(x) at a faster etch rate than it etches Si.Moreover, because oxidation rate of Si is much lower (sometimes 30 timeslower) than oxidation rate of SiGe, the channel layers 112 may not besignificantly etched by the channel release process.

In the present embodiment, since the inner spacer 180 is made of amaterial that has etching selectivity to that of the first semiconductorlayers 110, the inner spacer can protect the source/drain epitaxiallayers 190 from the etchant used in etching the first semiconductorlayers 110.

Referring to FIG. 16, a gate dielectric layer 202 is formed around eachchannel layers 112, and a metal gate electrode 204 is formed on the gatedielectric layer 202. The gate dielectric layer 202 and the metal gateelectrode 204 are collectively referred as to a metal gate structure200. The metal gate structure 200 extends in a Y direction which isperpendicular to the X direction and the Z direction. The exemplarysequential processes of the formation of the metal gate structure 200will be discussed in the following figures.

FIG. 17A to FIG. 21B are various cross-sectional views of a GAA deviceat different stages of fabrication, according to some embodiments of thedisclosure, in which “A” are the cross sectional views corresponding toline X2-X2 of FIG. 16, and “B” are the cross sectional viewscorresponding to line Y1-Y1 of FIG. 16. FIGS. 17A and 17B follow afterFIGS. 15A and 15B.

After the first semiconductor layers 110 are removed, interfacial layers206 are formed on surface of the channel region, e.g., the surface ofthe channel layers 112, and on the surface of the recessed fins 165. Theinterfacial layers 206 are formed of silicon oxide or silicon oxynitridegrown by a thermal oxidation process. For example, the interfacial layer206 can be grown by a rapid thermal oxidation (RTO) process or by anannealing process using oxygen.

Referring to FIGS. 18A and 18B, after the interfacial layers 206 areformed, a high-k gate dielectric layer 208 is formed along the gatesidewall spacers 172, the surface of the inner spacer 180, the surfaceof the ILD layer 194 and the interfacial layer 206, by a depositionprocess. In some embodiments, the high-k gate dielectric layer 132 mayinclude metal oxides. Examples of metal oxides used for high-k gatedielectric layer 132 include oxides of Li, Be, Mg, Ca, Sr, Sc, Y, Zr,Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu andmixtures thereof. The high-k dielectric layer 208 may be formed using asuitable process such as atomic layer deposition (ALD), chemical vapordeposition (CVD), physical vapor deposition (PVD) or other suitablemethod.

Reference is made to FIGS. 19A and 19B. A work function layer 210 isformed on the high-k gate dielectric layer 208. For example, the workfunction layer 210 is deposited to surround each of the channel layers(or nanosheets) 112. A portion of the work function layer 210 is formedvertically between adjacent channel layers (or nanosheets) 112 and fillsthe gap 113 between adjacent channel layers 112. In the presentembodiment, the work function layer 210 may be a p-type work functionlayer. Exemplary p-type work function materials include titaniumnitride, TaN, Ru, Mo, Al, WN, ZrSi₂, MoSi₂, TaSi₂, NiSi₂, WN, othersuitable p-type work function materials, or combinations thereof. Insome embodiments, the work function layer 210 is a nitride layer. Insome embodiments where the work function layer 210 is a titanium nitridelayer, the work function layer 210 may be formed by plasma enhancedatomic layer deposition (PEALD) by sequentially introducing atitanium-based precursor and nitrogen-based plasma as a reactant.Example of a titanium-based precursor include titanium organometallicprecursor such as tetrakisdimethyl titanium (Ti[N(CH₃)_(2]4), TDMAT). Insome embodiments, N₂ plasma may be used during the deposition process todissociate the precursor. In other words, the TDMAT is dissociated bythe N₂ plasma.

As compared to atomic layer deposition (ALD) method which uses athermally activated reaction, the deposition temperature is lower duringPEALD. The PEALD may have lower deposition temperatures (a lower thermalbudget) compared to their thermal counterparts (e.g., methods that useonly thermal energy for the dissociation of the precursors), since theplasma can provide additional “energy” that is available for thedissociation of precursors is necessary. Consequently, a similar thermalprocess may need to operate at a higher temperature if the samereactants are used. For example, a thermal (e.g., ALD) process oftitanium nitride deposition may operate at substrate temperatures ofgreater than 450° C. In some embodiments where the work function layer210 includes titanium nitride, the PEALD process of titanium nitridedeposition is performed at a substrate temperature below about 450° C.For example, the PEALD process is performed at a substrate temperatureof about 250° C. to about 450° C. and under a pressure of about 1 torrto about 20 torr. Since an atomic ratio of titanium to nitrogen (Ti/N)of titanium nitride is proportional to the formation temperaturethereof, by forming titanium nitride at a temperature of about 250° C.to about 450° C., the atomic ratio of Ti/N in the work function layer210 can be less than 1, which means nitrogen atomic percentage is higherthan titanium atomic percentage in the work function layer 210. In thiscase, the work function layer 210 can be interchangeably referred to asa nitrogen-dominant or nitrogen-rich TiN layer. For example, the atomicratio of Ti/N in the work function layer 210 is in a range from about0.6 to about 1. The threshold voltage can be controlled by the atomicratio of Ti/N in titanium nitride. Such reduced atomic ratio isbeneficial for decreasing the threshold voltage. In some embodiments,the decreasing amount of the threshold voltage is in a range from about35 mV to about 40 mV.

FIG. 22 is a chart illustrating flat band voltage (Vn) versuscapacitance equipment thickness (CET) with respect to the nitrogen-richTiN layer 210 in FIGS. 19A and 19B and a reference TiN layer (withatomic ratio of Ti/N equal to 1). Referring to FIG. 22, a Vn curve 1000of the nitrogen-rich TiN layer 210 and a Vn curve 1002 of the referenceTiN layer are depicted. The Vn curves 1000 and 1002 are given in termsof the flat band voltage (Vn) in voltage versus the capacitanceequipment thickness (CET) in nanometer. As shown in FIG. 22, a flat bandvoltage of the nitrogen-rich TiN layer 210 is higher than a flat bandvoltage of the reference TiN layer. That is, the flat band voltage ofthe nitrogen-rich TiN layer 210 shifts towards the p-type band-edge workfunction. Therefore, a threshold voltage of the nitrogen-rich TiN layer210 may be lower than a threshold voltage of the reference TiN layer.FIG. 23 is an atomic model of the nitrogen-rich TiN layer 210 in FIGS.19A and 19B (see FIGS. 19A and 19B). Referring to FIG. 23, according todensity-functional theory (DFT) calculation, nitrogen-rich (N-rich)titanium nitride is beneficial to p-type work function since it canprovide increased effective work function (eWf), for example, about 9meV/% or more.

FIG. 24 shows a schematic diagram of a sequence of the PEALD processaccording to some embodiments of the disclosure. Referring to FIG. 24,the formation of the nitrogen-rich TiN layer 210 is a cyclic process.The cyclic process includes one or more repetitions of a TDMAT providingstep 2000 and an N₂ plasma providing step 2002. The repetitions continueuntil the nitrogen-rich TiN layer 210 (see FIGS. 19A and 19B) of adesired thickness is obtained. For example, the titanium-rich TiNmaterial is merged into a continuous structure during the PEALD to formthe work function layer 210. In some embodiments, the number ofrepetitions of TDMAT providing step 2000 and N₂ plasma providing step2002 is about 10 to about 50.

Referring back to FIGS. 20A and 20B, a glue layer 212 is formed on andsurrounds the work function layer 210. The glue layer 212 may be used toincrease adhesion between the work function layer 210 and a subsequentlyformed metal fill layer (see FIGS. 21A and 21B) so as to prevent themetal fill layer from peeling or delaminating. The glue layer 212 is aconformal layer and is conformally formed over the work function layer210. In some embodiments, the glue layer 212 is a nitride layer. In someembodiments, the glue layer 212 is made of or includes tantalum nitride,titanium nitride, another suitable material, or a combination thereofand may be formed by CVD, ALD, PVD and/or other suitable process. Insome embodiments, the glue layer 212 and the work function layer 210include the same material with different stoichiometries such that theglue layer 212 and the work function layer 210 have an interfacetherebetween. For example, the glue layer 212 and the work functionlayer 210 include titanium nitride with different stoichiometries. Inother words, the Ti/N atomic ratio of TiN in the glue layer 212 isdifferent from the Ti/N atomic ratio of TiN in the work function layer210. For example, the Ti/N atomic ratio of TiN in the glue layer 212 isgreater than the Ti/N atomic ratio of TiN in the work function layer210. In some embodiments, the Ti/N atomic ratio of TiN in the glue layer212 is about 1. In some embodiments, the glue layer 212 has a thicknessless than a thickness of the work function layer 210.

In some embodiments, the glue layer 212 and the work function layer 210are formed by different methods. For example, the glue layer 212 isformed by ALD and the work function layer is formed by PEALD. The gluelayer 212 and the work function layer 210 are formed using differenttitanium-based precursors. For example, the glue layer 212 is formedusing titanium tetrachloride (TiC1 ₄) and the work function layer 210 isformed using TDMAT. A formation temperature of the glue layer 212 isdifferent from a formation temperature of the work function layer 210.For example, the formation temperature of the glue layer 212 is greaterthan the formation temperature of the work function layer 210. Inparticular, in some embodiments where the glue layer 212 includestitanium nitride formed using ALD, the ALD is carried out using titaniumtetrachloride (TiCl₄) and ammonia (NH₃) as precursors and argon (Ar) aspurging gas at a substrate temperature greater than about 350° C.

Referring to FIGS. 21A and 21B, the metal fill layer 214 is formedwithin a trench on the glue layer 212 between the gate sidewall spacers172. The metal fill layer 214 is deposited over the work function layer210. The metal fill layer 214 fills the opening 128. The metal filllayer 214 may include aluminum, tungsten, cobalt, copper, and/or othersuitable materials, and may be formed by CVD, PVD, plating, and/or othersuitable processes. In an embodiment, after the interfacial layers 206,the high-k gate dielectric layer 132, the work function layer 210, theglue layer 212 and the metal fill layer 214 are deposited, a CMP processis performed to planarize a top surface of the semiconductor device 10.

Based on the above discussion, it can be seen that the presentdisclosure offers advantages. It is understood, however, that otherembodiments may offer additional advantages, and not all advantages arenecessarily disclosed herein, and that no particular advantages isrequired for all embodiments. One advantage is that the atomic ratio ofTi/N in the work function layer can be less than 1 by forming the workfunction layer at a temperature of about 250° C. to about 450° C.Another advantage is that such reduced atomic ratio is beneficial fordecreasing the threshold voltage of an amount in a range from about 35mV to about 40 mV.

In some embodiments, a method of forming a semiconductor device includesforming a fin structure having a stack of alternating firstsemiconductor layers and second semiconductor layers over a substrate,the first semiconductor layers and the second semiconductor layershaving different compositions, forming a dummy gate structure across thefin structure, forming gate spacers on opposite sidewalls of the dummygate structure, respectively, removing the dummy gate structure to forma gate trench between the gate spacers, etching the first semiconductorlayers in the gate trench, such that the second semiconductor layers aresuspended in the gate trench to serve as nanosheets, depositing a firsttitanium nitride layer surrounding each of the nanosheets, an atomicratio of titanium to nitrogen of the first titanium nitride layer isless than 1, and forming a metal fill layer over the first titaniumnitride layer.

In some embodiments, a method of forming a semiconductor device includesforming a fin structure having a stack of alternating firstsemiconductor layers and second semiconductor layers over a substrate,forming a dummy gate structure across the fin structure, forming gatespacers on opposite sidewalls of the dummy gate structure, respectively,removing the dummy gate structure to form a gate trench between the gatespacers, etching the first semiconductor layers in the gate trench, suchthat the second semiconductor layers are suspended in the gate trench toserve as nanosheets, performing a plasma deposition process to formtitanium nitride materials between each of the nanosheets, wherein theplasma process is a cyclic process comprising one or more repetitions ofa tetrakisdimethyl titanium (TDMAT) providing step and an N₂ plasmaproviding step, and depositing a metal fill layer over the titaniumnitride materials.

In some embodiments, a semiconductor device includes a plurality ofnanosheets and a gate structure. The nanosheets extend in a firstdirection above a semiconductor substrate and are arranged in a seconddirection substantially perpendicular to the first direction. The gatestructure extends in a third direction perpendicular to both the firstand second directions. The gate structure surrounds each of theplurality of nanosheets. The gate structure includes a first titaniumnitride layer, a second titanium nitride layer and a metal fill layer.The first titanium nitride layer surround each of the plurality ofnanosheets. The second titanium nitride layer surrounds the firsttitanium nitride layer. The second titanium nitride layer has an atomicratio of titanium to nitrogen greater than that of the first titaniumnitride layer. The metal fill layer surrounds the second titaniumnitride layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method of forming a semiconductor device,comprising: forming a fin structure having a stack of alternating firstsemiconductor layers and second semiconductor layers over a substrate,the first semiconductor layers and the second semiconductor layershaving different compositions; forming a dummy gate structure across thefin structure; forming gate spacers on opposite sidewalls of the dummygate structure, respectively; removing the dummy gate structure to forma gate trench between the gate spacers; etching the first semiconductorlayers in the gate trench, such that the second semiconductor layers aresuspended in the gate trench to serve as nanosheets; depositing a firsttitanium nitride layer surrounding each of the nanosheets using a plasmaenhanced atomic layer deposition (PEALD) that includes sequentiallyintroducing a first titanium-based precursor and a nitrogen-basedplasma, wherein the nitrogen-based plasma is used during depositing thefirst titanium nitride layer to dissociate the first titanium-basedprecursor, an atomic ratio of titanium to nitrogen of the first titaniumnitride layer is less than 1, and the step of depositing the firsttitanium nitride layer fills a gap between adjacent two of thenanosheets; and forming a metal fill layer over the first titaniumnitride layer.
 2. The method of claim 1, further comprising: forming asecond titanium nitride layer over the first titanium nitride layer,wherein the second titanium nitride layer and the first titanium nitridelayer are formed by different deposition methods.
 3. The method of claim2, wherein an atomic ratio of titanium to nitrogen of the secondtitanium nitride layer is different from the atomic ratio of titanium tonitrogen of the first titanium nitride layer.
 4. The method of claim 2,wherein the first titanium nitride layer is formed at a temperaturelower than forming the second titanium nitride layer.
 5. The method ofclaim 2, wherein the second titanium nitride layer is formed byintroducing a second titanium-based precursor different from the firsttitanium-based precursor.
 6. The method of claim 5, wherein one of thefirst titanium-based precursor and the second titanium-based precursoris organic.
 7. The method of claim 1, wherein the atomic ratio oftitanium to nitrogen of the first titanium nitride layer is greater thanor equal to 0.6.
 8. The method of claim 1, wherein a portion of thefirst titanium nitride layer is vertically between the adjacentnanosheets.
 9. The method of claim 2, wherein the second titaniumnitride layer is formed by atomic layer deposition (ALD) using titaniumtetrachloride (TiCl₄) and ammonia (NH₃) as precursors and argon (Ar) aspurging gas.
 10. A method of forming a semiconductor device, comprising:forming a fin structure having a stack of alternating firstsemiconductor layers and second semiconductor layers over a substrate;forming a dummy gate structure across the fin structure; forming gatespacers on opposite sidewalls of the dummy gate structure, respectively;removing the dummy gate structure to form a gate trench between the gatespacers; etching the first semiconductor layers in the gate trench, suchthat the second semiconductor layers are suspended in the gate trench toserve as nanosheets; performing a plasma enhanced atomic layerdeposition (PEALD) process to form titanium nitride materials betweeneach of the nanosheets, wherein the PEALD process comprises a pluralityof PEALD cycles, each of the PEALD cycles includes a tetrakisdimethyltitanium (TDMAT) pulsing step and an N2 plasma step dissociating TDMATpulsed in the TDMAT pulsing step; and depositing a metal fill layer overthe titanium nitride materials but not into a gap between adjacent twoof the nanosheets.
 11. The method of claim 10, wherein the titaniumnitride materials are merged into a continuous structure during thePEALD process.
 12. The method of claim 10, wherein the PEALD process isperformed at a substrate temperature in a range from about 250° C. toabout 450° C.
 13. The method of claim 10, further comprising: afterperforming the PEALD process, forming a conformal layer comprisingtitanium nitride wrapping around the titanium nitride materials.
 14. Themethod of claim 13, wherein the conformal layer and the titanium nitridematerials have an interface therebetween.
 15. The method of claim 13,wherein the titanium nitride materials have a stoichiometry differentfrom a stoichiometry of the conformal layer.
 16. The method of claim 13,wherein a formation temperature of the conformal layer is different froma formation temperature of the titanium nitride materials.
 17. Themethod of claim 13, wherein a formation temperature of the conformallayer is greater than a formation temperature of the titanium nitridematerials.
 18. A semiconductor device, comprising: a plurality ofnanosheets extending in a first direction above a semiconductorsubstrate and arranged in a second direction substantially perpendicularto the first direction; and a gate structure extending in a thirddirection perpendicular to both the first and second directions, thegate structure surrounding each of the plurality of nanosheets, whereinthe gate structure comprises: a high-k gate dielectric layer surroundingeach of the plurality of nanosheets; a first titanium nitride layer overthe high-k gate dielectric layer, wherein an atomic ratio of titanium tonitrogen of a portion of the first titanium nitride layer in contactwith the high-k gate dielectric layer is less than 1 and greater than0.6; a second titanium nitride layer surrounding the first titaniumnitride layer, the second titanium nitride layer has an atomic ratio oftitanium to nitrogen greater than that of the first titanium nitridelayer; and a metal fill layer surrounding the second titanium nitridelayer and absent between adjacent two of the plurality of nanosheets.19. The semiconductor device of claim 18, wherein the atomic ratio oftitanium to nitrogen of the second titanium nitride layer is about 1.20. The semiconductor device of claim 18, wherein the second titaniumnitride layer has a thickness less than a thickness of the firsttitanium nitride layer.